This invention relates to devices which are useful in converting the rotational movement of a threadless shaft into linear motion imparted from the threadless shaft to the device. Such a device is fully described in basic form in U.S. Pat. No. 3,272,021 issued Sept. 13, 1966 and which is incorporated herein by reference. The invention relates to improvements in construction which result in a performance improvement to the device.
The device is shown in the aforesaid patent finds one application as a driver for automobile power windows. The device is comprised of two blocks which are drawn toward each other by screws acting through the springs. The blocks are each formed with a U-shaped track which after assembly forms a curvilinear bore. There are roller bearings at each end of the device; and, in particular each block has roller bearings associated with itself. In the above-described device, four roller bearings are associated with each block, two at each end of the block. Further, since there are two blocks to the device, the device includes a total of eight roller bearings. The roller bearings are each skewed with respect to the shaft to establish a lead angle which determines the amount of travel of the device as the shaft rotates.
The prior art further includes a similar device to the previously described patented device which is sold by the Barry Wright Corporation under the registered trademark Roh'lix. In that device, there are a total of six bearings instead of the eight bearings previously mentioned. The orientation of the bearings in the device is such that each block contained a total of three bearings, two at one end and one at the other end. Upon assembly, the blocks are assembled so that there are three bearings at each end of the block, two on one block and one on the other. This construction required that the blocks be formed in complementary fashion with opposite vertical faces such as a lap joint. This device, although well received in the market place, experiences certain undesirable characteristics which are addressed by the present invention.
One such undesirable characteristic is the lack of position repeatability. For example, because of the manner in which forces act on the device for each shaft revolution, it travels a different distance in one direction on the shaft than in the other direction. Likewise, there is a thrust variation caused by directional reversal so that thrust in one direction is different than thrust in the opposite direction.
The prior art further includes a similar device described in basic form in U.S. Pat. No. 4,411,166. In this device, the two blocks are pivotably associated with each other. Two bearings are placed on a pivoting block and four bearings are placed on a fulcrum block. The undesirable characteristics shared by this device and others of the prior art are their sensitivity to manufacturing tolerances affecting bearing placement and sensitivity to variations in the shaft size and shaft geometry upon which it will be disposed. The result of these sensitivities is an inordinate effect on the relationship between the drive shaft and mounting surface of the actuator.
It is an object of this invention to provide a device for converting rotational shaft motion into linear movement of the device which will have improved repeatability with respect to directional travel.
It is another object of this invention to provide a device for converting rotational shaft motion into linear movement of the device which will have less variation in thrust based upon directional travel.
It is further an object of this invention to produce device of the type previously mentioned which will improve load sharing among the bearings including the periods during static loads.
Another object of this invention is to produce a device which is less sensitive to the dimensional variations resulting from its manufacture and variations in shaft dimensions and geometry, thus improving the relationship between the drive shaft and the mounting surface of the device.
It is also an object of this invention to produce a device for converting rotational motion into linear movement where the manufacturing tolerances which position the bearings and where variations in drive shaft size and geometry have less effect on the relationship between mounting components.
Further, it is an object of the present invention to provide a device wherein the component blocks are assembled in such a manner as to minimize variations in the positioning of the blocks during final assembly from the positioning of blocks during the original machining process.
Other objects and advantages of the invention will become apparent from the following detailed description of the invention and the appended drawings.